1. Field of the Invention
The present invention relates to a method for depositing another thin film on an oxide thin film having a perovskite crystal structure, more specifically to a method for depositing another thin film on an oxide thin film having a perovskite crystal structure formed on a substrate with little mutual diffusion at an interface between the thin films.
2. Description of Related Art
Oxide superconductors which have been recently advanced in study have higher critical temperatures than those of metal superconductors, so that they are more practical. For example, Y-Ba-Cu-O type oxide superconductor has a critical temperature higher than 80 K. and it is announced that Bi-Sr-Ca-Cu-O type oxide superconductor and Tl-Ba-Ca-Cu-O type oxide superconductor have critical temperatures higher than 100 K. These oxide superconductors have perovskite type crystal structures.
In order to apply the oxide superconductors to superconducting devices, it is necessary to form oxide superconductor thin films. The superconductor thin films are usually deposited on single crystal substrates, for example a MgO substrate, a SrTiO.sub.3 substrate, a YSZ (yttrium stabilized zirconia) substrate, or etc., by various sputterings, an MBE (molecular beam epitaxy), a reactive co-evaporation, a CVD (chemical vapor deposition). The superconductor thin films grow epitaxially on these substrate.
It is also necessary to stack other thin films on the oxide superconductor thin films for manufacturing superconducting devices. For example, a tunnel type Josephson junction comprises a stacked structure of a first superconducting layer, an thin insulator layer and a second superconducting layer stacked in the named order. Therefore, if an oxide superconductor is applied to the tunnel type Josephson junction device, a first oxide superconductor thin film, an insulator thin film and a second oxide superconductor thin film should be stacked in the named order.
A superconducting field effect device, one of three-terminal superconducting devices has a superconducting channel and a gate electrode formed on the superconducting channel through a gate insulating layer. If a superconducting field effect device is manufactured by using an oxide superconductor, it is necessary to stack an oxide superconductor thin film, an insulator thin film and a conductor film.
In addition, in order to fabricate a superconducting multi-layer wiring structure by using an oxide superconductor, oxide superconducting thin films and insulator thin films should be alternately stacked.
In the above superconducting devices and elements, high quality thin films are required. Namely, oxide superconductor thin films and other thin films preferably have not only excellent properties but also high crystallinity. It is more preferable that the thin films are formed of single crystals. If one of the thin films is formed of polycrystals or amorphous, the device or the element may have a low performance or may not operate.
Additionally, in the above superconducting devices and elements, conditions of interfaces between the thin films are also important. It is preferable that there is no mutual diffusion at the interfaces and the interfaces are formed sharply. In particular, if constituent elements of the nonsuperconductor thin films diffuse into the oxide superconductor thin films, superconducting properties of the oxide superconductor thin films are considerably lowered.
In a prior art, when nonsuperconductor thin films are deposited on oxide superconductor thin films formed on substrates, substrate temperatures are equal to or a little lower than ones under which the oxide superconductor thin films has been formed. For example, A. Walkenhorst et al. disclosed in Applied Physics Letter, vol. 60, No. 14, pp. 1744-1746 that they deposited SrTiO.sub.3 thin films under substrate temperatures of 750.degree.-800.degree. C. on Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-.delta. thin films which had been deposited on SrTiO.sub.3 substrates under the same substrate temperatures of 750.degree.-800.degree. C.
However, if a nonsuperconductor thin film is deposited on an oxide superconductor thin film under such a high substrate temperature, significant mutual diffusion at the interface is caused. Since, a high temperature of the interface is maintained for a long time. By this, both the oxide superconductor thin film and the nonsuperconductor thin film are degraded so as to have poor qualities and properties.
If the nonsuperconductor thin film is deposited under a lower substrate temperature in order to avoid the mutual diffusion at the interface, the nonsuperconductor thin film becomes low crystallized or may be formed of amorphous so as to have poor properties.